Posted by
Geoffrey Mackenzie-Kennedy-2 on
Dec 08, 2005; 1:42pm
URL: http://elecraft.85.s1.nabble.com/Clever-antenna-tuning-ideas-wanted-tp384403p384408.html
Vic Rosenthal wrote:
> OK, here is what I want to do:
>
> I have a 33' (10m) vertical with the base about 10' (3.3m) above ground.
> It has four 8' (2.4m) radials which are connected to the feedpoint through
> a coil which resonates the system on 7 MHz. It works as well or better
> than any vertical that I've used on 40m -- low SWR and good results. It's
> fed through a relatively short (20', 6m) length of RG-213.
>
> I would like to use it on 80m and 30m as well. My first try was to simply
> use the KAT100 to tune it. It tunes fine on both bands (1:1 at the rig),
> but results are fair-to-poor on 30 and poor-to-worthless on 80.
>
> On 30m, the coax losses are reasonable (the SWR on the coax is about 8 to
> 1, but it's very short). I suspect the problem is that the radial ground
> system is very poor on this band (on the other hand, the long vertical
> radiator should provide a slight amount of gain). So I may try
> paralleling a set of radials tuned for 30m. They will also be short, but
> resonated with a coil or stub. Does this sound like a good
> analysis/solution?
...................................................................................................
Hi Vic,
Do you have EZNEC or any other form of NEC? I had a quick look at what you
have, making many assumptions such as element / radial diameters, slope of
radials, azimuth of radials, ground type etc etc etc, and I think that on
30m the vertical element + radials + radial loading coil looks more like an
offset fed "dipole" with a drive point impedance around 120 +j something.
The vertical pattern is still good, with a little gain over a monopole. On
80m the vertical pattern broadens with the take-off angle increasing to 26 -
28 degrees, but still with a deep null at 90 degrees although the gain is
poor. The drive point impedance is VERY low and Complex. The actual
orientation and slope of the existing radials will have great effect on all
bands, and some form of additional 80m radials could be required if the
vertical portion is not top loaded.
If you want to add or remove parts when changing bands, you might like to
consider "passive switching". Parallel tuned traps are one form of
"switch", but there is also the series tuned form. For example if you wanted
to remove the tuned coil used with the radials on 40m when going to 30m, a
series LC tuned to 30m placed across the 40m "radial coil" would do this.
Some fiddling is needed because there would be some interaction between the
40m and 30m circuits. All sorts of games can be played with passive
switching although there is a limit as to how many bands can be handled by
one switching module, and you don't want to introduce loss by using dinky
little coils etc.
If you would like to continue, I suggest that we do it off-list to avoid
using List Bandwidth.
73,
Geoff.
GM4ESD
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